The present invention relates to bulk starters for cheesemaking and, in particular, to a bulk starter which contains both Lactobacillus (rod) and Streptococcus (cocci) organisms. In the commercial production of cheese, large vats of milk are treated with a milk clotting agent such as rennin and inoculated with lactic acid producing bacteria such as Streptococcus lactis, S. cremoris, S. thermophilus, Lactobacillus bulgaricus and L. helviticus. These bacteria are capable of fermenting lactose or similar carbohydrates to produce lactic acid.
The bacteria is generally propagated from a mother culture in large enough quantities of aqueous medium to produce a bulk starter which can then be used for fermenting the final batch of milk to produce cheese as the final product. In a typical procedure, a bulk starter medium is prepared by dissolving a dry powder comprising a nutrient base and a growth stimulant, together with alkali metal phosphate and/or polyphosphate phage control agents in water to a level of about 8% solids. The solution is sterilized and cooled whereupon it is inoculated with the lactic acid producing bacteria which is then allowed to incubate. After incubation, the "ripened" starter is added to the cheese milk at a level of from about 1/2 to 2 weight percent. The cheese milk at this point will have a pH of approximately 6.6, however, as the bacteria grow, the acid they produce will gradually lower the pH.
In the manufacture of Italian cheeses such as mozzarella it is the usual practice to employ a combination of starter bacteria. For example, a Streptococcus thermophilus together with one or more lactobacilli such as Lactobacillus bulgaricus may be used. Streptococci are generally referred to as "cocci" and the lactobacilli as "rod" bacteria because of their appearance when examined under a microscope. In the manufacture of Italian cheese, the ratio of cocci to rods in the starter should be from about 4:1 to 1:4 for best results in terms of rheological properties and flavor of the resulting cheese. The most desirable ratio for certain operations is about 1:1 and in others an excess of cocci is preferred.
As mentioned above, the growth of the lactic acid producing bacteria will gradually lower the pH of the starter system due to the formation of lactic acid. Too great a lowering of the pH is detrimental because the bacteria's growth is inhibited and cell loss can occur in the case of excess acidity. Accordingly, excess acidity is neutralized during the bacteria's growth cycle either internally such as by incorporating an insoluble buffering agent into the starter media as disclosed in U.S. Pat. No. 4,282,255 or externally by introducing a base (e.g. ammonium hydroxide). In the case of an Italian type cheese starter containing cocci and rods, pH control takes on additional significance because the rods, i.e. lactobacilli, grow at a faster rate than do the cocci at lower pH values. Since the lactobacilli are more acid tolerant than are the cocci, if the culture is maintained at a relatively low pH the rods will grow more rapidly whereas reproduction of cocci is favored at a higher pH. In addition, the rods are more acid tolerant and will not be killed as readily as the cocci at a relatively low pH. This phenomena is discussed in U.S. Pat. No. 4,622,304 which discloses incubating a mixture of rod and coccus organisms without neutralization of acid until the pH of the growth medium drops to a level of from 3.9 to 5.5 and then quickly raising the pH by at least 1 unit with further incubation of the microorganisms until the ratio of cocci to rods is from about 2:1 to 5:1. This system, sometimes referred to as one step neutralization, differs from conventional external pH control where the media's pH is simply maintained at a pre-selected level and is said to result in an increase of the population of organisms in the finished starter over that which is obtainable using internal pH control. However, this system is not totally satisfactory for obtaining the desired ratio of cocci to rods because in this procedure, the pH of the incubating starter traverses a wide range and is never maintained at a pre-selected pH long enough for the rod/cocci ratio to stabilize.
In conventional external methods of neutralization, the pH of the incubating starter is allowed to drop to a pre-determined pH, e.g. 5.0, and then raised about 0.20 pH units. The incubation is allowed to proceed until the pH again drops to 5.0 and then raised to about 5.2. This procedure is repeated until the available nutrients in the medium are utilized. A graph of pH -vs- time for this method would be saw-toothed in appearance. This procedure maintains the pH of the medium in a single narrow range for the entire growth period of the starter and produces a starter with a single coccus:rod ratio depending on the pH range that is maintained. However, the flexibility of this method for producing a wide range of ratios is limited. If the pH is held below 5.0 in order to increase the rod population, the cocci undergo acid damage resulting in a less active starter and increased inoculation rates in the cheese milk. If the pH is maintained at a level of above 5.5 the coccus population are not acid damaged, however, the starter lacks suitable rod population for producing cheese of optimum quality.
U.S. Pat. No. 3,998,700 describes the optimum ratio of cocci to rods for the manufacture of mozzarella as being from about 3:1 to 5:1 but indicates that some latitude is acceptable as long as the ratio is retained in a range of from 2.5:1 to 5.5:1. This patent discloses a media particularly suited for propagating a cocci/rod starter culture which media comprises acid whey solids and sweet whey solids together with non-fat dry milk.
The growth rates of bacteria are also affected by temperature. The growth rate of Lactobacillus sp (rods) is greater than that of streptococcus sp (cocci) at higher temperatures. Conversely, the cocci will propagate faster than the rods at lower temperatures. Thus, combining high temperature with low pH will favor rod growth whereas raising the pH and lowering the temperature of the culturing media will favor the propagation of cocci. While temperature variations will affect the relative growth rate of rod and cocci bacteria, it is pH control that is of primary significance in controlling these growth rates.
It would be desirable and it is an object of the 5 present invention to provide a method for the incubation of a cheese starter system containing cocci and rod type bacteria in which the ratio of the two types of bacteria can be accurately controlled while increasing the total number of organisms in the finished starter.